The theory for a light-emitting diode (LED) to emit light is that when a forward voltage is applied to a p-n junction, the electrons are driven from the n-type semiconductor and the holes are driven from the p-type semiconductor, and these carriers are combined in an active region to emit light. The efficiency of an LED depends on the Internal Quantum Efficiency (IQE). The IQE depends on the combining rate of the electrons from the n-type semiconductor and the holes from the p-type semiconductor, and is reduced by the built-in electric field.
The built-in electric field is an effect due to the existence of polarized electric charges which arise from a piezoelectric characteristic and a spontaneous polarization as a result of the characteristics of the LED's materials. The built-in electric field is particularly significant for materials comprising a composition of AlxInyGa(1-x-y)N because of the wurtzite or zinc blende structure of the material. For example, an LED which adopts AlxInyGa(1-x-y)N films grown along the polar c-direction of a sapphire substrate suffers from the undesired built-in electric field.
FIG. 1 shows the schematic diagram of the energy band of an active region of an unbiased conventional AlxInyGa(1-x-y)N light-emitting diode. The active region comprises two GaN barrier layers 103b with an InGaN well layer 103w therebetween. All layers are of wurtzite crystal structure. The horizontal axis represents position in the active region along a line substantially in parallel to a direction which the layers are stacked. The interfaces between the layers are indicated by dashed lines. The vertical axis represents the energy of the conduction band (marked by Ec) and the energy of the valence band (marked by Ev) for the layers. The difference between energy of the conduction band and energy of the valence band is called the energy band gap.
In the absence of a spontaneous polarization, piezoelectric fields, and an externally applied bias, the conduction band Ec and valence band Ev are substantially flat within each layer. However, as shown in the figure, the built-in electric field due to the existence of polarized electric charges tilts the bands. The tilt adversely affects the IQE, and as a result, the electron wave function 103we and the hole wave function 103wh are concentrated on opposite sides of InGaN well 103w. The spatial overlap of these wave functions is therefore reduced and leads to a decrease of the probabilities of combination of electrons and holes. Hence, the IQE or the efficiency of an LED is reduced.